As far as the cost of heating the water you use there is no difference.
The cost of raising the temperature of a gallon of water from 70 to 120 degrees F is the same no matter what. However if you start taking longer showers because you have more water available then it will cost you more.
In todays water heaters most have a good amount of foam insulation. This helps prevent standby heat loss which has been reduced to a very small amount. The only difference you will see is the difference in loss because of the larger surface area of the tank. So you are looking at an inconsequential amount of an inconsequential amount....
Bottom line is properly size the water heater to match your hot water needs.
In your situation you may want to look at heat pump water heaters. Ge Heat Pump Water Heater